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The structural basement in the Rocky Mountains foreland deforms by faulting and rigid-body rotations. The faults at the interface between the sedimentary and crystalline rocks can be anything from low-angle reverse to normal faults. Despite the range of geometries, the total assemblage of faults and rotations is best explained by a movement system that is dominated by vertical motions along faults, many of which are curved in cross section. The first causes deep within the crust are not sufficiently well documented in the geophysical record to justify a firm interpretation. However, there are certain conditions recorded in the geologic history of the region and in the surface structures that place constraints even on speculations. With these constraints, vertical movements seem more likely to dominate than horizontal movements. The sedimentary layers are deformed primarily by forced folding. Their final geometry is a product of several parameters such as welding and stratigraphic make-up. Measurements on natural folds demand that the section either thinned or detached. Detachment without appreciable thinning further requires that (1) large displacements occur within the sedimentary section and (2) at the termination of these folds, the movement must be in several directions. Geologists’ intuition as to how the layered rocks achieved their shapes is not always correct, but field data combined with experimental and theoretical data provide a basis for understanding these folds. It is concluded that the structural style in the Rocky Mountains foreland is not unique, but rather it is only an excellent example of a more universal class of deformation, namely, forced folding.

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